1. Field of the Invention
The present invention relates to a method of cleaning a doping system which is used during fabrication of semiconductor integrated circuits or the like.
2. Prior Art Techniques
When n-type or p-type doped regions are formed in a semiconductor during fabrication of a semiconductor integrated circuit or the like, it is known to accelerate ions of impurities of the n- or p-type at a high voltage, for implanting the impurities into the semiconductor. Especially, the method of separating ions in terms of mass-to-charge ratio is known as the ion implantation and has enjoyed wide acceptance in semiconductor integrated circuit fabrication processes.
Another known method consists of generating a plasma having an n- or p-type impurity and accelerating the ions contained in the plasma at a high voltage so as to introduce the ions into a semiconductor. This method is known as the ion doping or plasma doping.
For example, where boron is implanted as a p-type impurity, a plasma is generated by RF discharge or other method within a gas such as diborane (B.sub.2 H.sub.6) which is a boronic compound. Then, a high voltage is applied across the plasma. The ions having boron are extracted and lodged into a semiconductor.
Generally, the former method needs complex and expensive equipment and is capable of processing only substrates having small areas. On the other hand, the latter method requires only simple and inexpensive equipment and is able to process large-area substrates. Therefore, it is considered that the latter method is used for manufacture of active matrix liquid crystal displays having large-area substrates to be processed.
However, in the equipment for the latter method, the dopant elements adhere to the inner wall of the chamber and so it is necessary to clean the inner wall of the chamber frequently. Especially, where lightly doped n- or p-type regions should be formed by introducing a trace amount of impurity, the characteristics of the finished semiconductor device are affected materially by impurities of dissimilar conductivity types adhering to the inner wall of the chamber. Consequently, it has been necessary to clean the chamber frequently, in order to do quite light doping with high reproducibility.
The cleaning has been performed by wet-etching processing. For this reason, whenever the equipment is cleaned, the chamber is opened to atmosphere. It takes a very long time to evacuate the chamber up to a sufficiently high degree of vacuum. Since the equipment is a doping machine, if the constituents of the atmosphere adhere to the inner wall of the chamber, the constituents are mixed into the semiconductor during doping. Hence, it is necessary to completely degas the chamber. The produced problem is that the operating efficiency of the equipment deteriorates.
For these reasons, dry cleaning processes in which the chamber is not required to be opened to atmosphere are necessary. In the prior art dry cleaning processes, a plasma is generated inside a chamber and thus elements adhering to the inner wall are removed by sputtering or chemical reactions. The problem with these prior art processes is that the chamber cannot be cleaned sufficiently up to its corners, because of spread of the plasma and because the plasma can be generated only within a limited region inside the doping equipment.
FIG. 1 schematically shows an ordinary ion doping system. This system comprises a chamber 1 which is shown to be cylindrical although the chamber 1 can assume various forms. Pipes for supplying gases used for doping such as diborane (B.sub.2 H.sub.6) and phosphine (PH.sub.3) are connected to the chamber. The doping system is further equipped with means for evacuating the inside of the chamber. In use, it is common practice to dilute the diborane and phosphine with hydrogen or the like. The inside of the chamber is partitioned into three major regions: a plasma source, an ion acceleration region, and an ion bombardment region.
The plasma source, indicated by numeral 3, is mounted near the top of the chamber and acts to generate a plasma by RF discharge or other method. The produced ions are accelerated toward the ion acceleration region located under the plasma source. The accelerated ions are directed to a sample which is placed on a sample stage 2 within the ion bombardment region. Thus, the sample is doped with impurities.
The ion acceleration region contains a plurality of slit-like electrodes as shown. A voltage is applied across the electrodes to extract and accelerate the ions. The slit-like electrodes include electrodes 4 and 5 that serve to extract the ions under the control of a power supply 8. A high voltage is applied between the electrode 5 and another electrode 7 by another power supply 9. Ions are accelerated within this region. If necessary, a reverse voltage can be applied to a further electrode 6 by a power supply 10 to decelerate the ions.
As can be seen from the figure, in the ordinary ion doping system, the region for producing a plasma is confined to the plasma source 3. Therefore, it is totally impossible to diffuse the plasma onto the whole inner wall of the chamber. However, the system is designed so that dopant ions can reach the doping region. Furthermore, the flow of ions has only a small degree of directivity. Therefore, the possibility that the dopant ions adhere to the inner wall or electrodes which are located in the ion path is very high. When the adhering impurities are subjected to another doping step, the impurities are ejected by ions and again introduced into the sample.
Consequently, dry cleaning using a plasma produces only a temporary effect. It has been necessary to clean the equipment by wet etching periodically.
In view of the foregoing problems, the present invention has been made. It is an object of the present invention to provide a method of cleaning up the inside of the chamber of an ion doping system by dry cleaning, especially by dry cleaning of boron.